Effects of Cations on Structural Transformation of Layered Manganese Oxides during Oxidation of Fulvic Acid.

Partially reductive dissolution of layered manganese oxides (birnessite) enriches the oxide structure with trivalent manganese (Mn(III)). Naturally occurring birnessite contains abundant metal cations. We examined the effects of the presence of Zn²⁺, Mg²⁺ or Ca²⁺ and ionic strength (0 mM versus 50 mM NaCl) on the structural transformation of birnessite (d-MnO₂) by reaction with fulvic acid (FA) at pH 8 and FA/MnO₂ mass ratios (R) of 0.1 or 1 over 25 d under anoxic conditions. Both the higher ionic strength and the presence of the divalent cations enhanced production of dissolved Mn and more strongly, FA adsorption and fractionation. Both Mg²⁺ and Ca²⁺ favor the Mn(III) formation in the layers and on the vacancies of reacted d-MnO₂ while Zn²⁺ shows an opposite effect. The presence of these cations also results in less Mn(II) adsorbed on vacancies. In addition, minor feitknechtite and hausmannite formed in the presence of Ca²⁺ or Mg²⁺. In the presence of Zn²⁺, no new phases formed at R = 0.1; however, at R = 1, Zn-substituted hausmannite formed extensively at the late stage. Our results indicate that the Mn(III) enrichment in layered Mn oxides during partial reductive disolution follows the same mechanism as that during biogenic oxidative precipitaton, i.e., the comproportionation between vacancy‑adsorbed Mn(II) and Mn(IV) in the layers. Ca²⁺, Mg²⁺ and high ionic strength all stabilize Mn(III) in the layers while Zn²⁺ competes with Mn(II) for vacancies, respectively enhancing and decreasing Mn(III) production. This work also highlights that Mn oxides can effectively stabilize organic carbon in the Mn-rich sediments of salty environments.